1. Field of the Invention
The present invention relates to a method for annealing (laser annealing) a thin film semiconductor by irradiating a laser light. The objects of laser annealing include to crystallize an amorphous thin film, to improve the crystallinity of a crystalline thin film, to activate impurity elements for imparting conductivity, and the like.
2. Description of the Related Art
In recent years, a technique which comprises forming a thin film semiconductor on a glass substrate and then fabricating a thin film transistor by using the thus obtained thin film is known. This technique is essential for the fabrication of an active matrix liquid crystal display device.
An active matrix liquid crystal display device comprises pixel electrodes provided in a matrix-like arrangement and thin film transistors provided to each of the pixel electrodes in order to control the charge that is input and output from the pixel electrode.
In fabricating the active matrix liquid crystal display device, several hundred thousand of thin film transistors must be integrated in a matrix-like arrangement.
Thin film transistors utilizing a crystalline silicon film are capable of yielding high performance, and are preferred for use in the liquid crystal display device. When a crystalline silicon film is used, in particular, peripheral drive circuits using thin film transistors can be constructed on the same glass substrate. Thus, an advantageous constitution, which enables a more compact device at a simpler fabrication process at a lower fabrication cost, etc., can be implemented.
However, an active matrix liquid crystal device at present has problems of causing uneven display or forming stripe patterns in the display. Especially, the stripe patterns are particular in a liquid crystal display device fabricated through a laser annealing process, and they considerably impair the visual appearance of the displayed image.
The stripe patterns differ from point defects and line defects in that they become visually perceptible depending on the drive conditions of the liquid crystal display device. Thus, the present inventors assumed that this phenomena differs from the permanent defects attributed to, for example, the destruction of thin film transistors and the formation of short circuit in the wirings and the like.
Then, as a result of analyzing the liquid crystal display device from various viewpoints, it has been found that the fluctuation in ON current (the current which generates in selecting a pixel electrode) greatly influences the generation of stripe patterns.
For instance, when a thin film transistor is selected in an active matrix liquid crystal display device, an ON current generates between the source region (connected to a data line) and the drain region (connected to a pixel electrode) of the active layer as to realize a particular state (charged state) in which a certain voltage is applied to the liquid crystal.
Thus, in case the ON current is extremely small, a problem may happen that the charge is insufficient for a pixel electrode. In such a case where the saturated charge is not attained, it becomes impossible to realize the desired gray-scale display, and those pixel regions with insufficient display are observed as stripe patterns.
Furthermore, there occurs a phenomenon of causing slight drop in the voltage written in the pixel electrode immediately after a thin film transistor is switched from an ON state to an OFF state (or from an OFF state to an ON state). The fluctuation in voltage is called as a xe2x80x9cfield through voltagexe2x80x9d.
The field through voltage is another factor causing stripe patterns, because the charge stored in the pixel electrode also changes with the field through voltage.
However, in general, the field through current is relaxed by a compensation current which generates between the source/drain (hereinafter referred to as xe2x80x9ca field-through compensation currentxe2x80x9d). The field-through compensation current is a current that generates within a short period of time in switching the thin film transistor from an ON state to an OFF state (or reverse).
The present inventors analyzed the trial-fabricated thin film transistor, and as a result, it has been found that, with increasing ON current, the field-through compensation current increases, i.e., that the field-through voltage becomes more relaxed.
The analyzed results above can be summarized as follows. That is, the long-unsolved problem of the generation of stripe patterns in a liquid crystal display device is attributed to the fluctuation in ON current of a thin film transistor, and the best solution of the problem is to overcome the fluctuation in ON current.
Furthermore, the present inventors simulated the generation of stripe patterns ascribed to insufficient charging described above by means of simulation. The simulation was performed by calculating the time necessary for charging 99.6% or more of the pixel capacitance of about 0.2 pF (a total capacitance of a capacitance of the liquid crystals and the auxiliary capacitance).
Based on the fact that the fly-back time in VGA is 5 xcexcs, and including margin, the results were evaluated by judging whether the pixel capacitance can be charged in a period of 2 xcexcs or not.
As a result, it was confirmed that an ON current (at a drain voltage Vd=14 V and a gate voltage Vg=10 V) of 3 xcexcA or higher is necessary in case of a thin film transistor with a threshold voltage of about 2 V.
In the light of the aforementioned circumstances, the present inventors came to a conclusion that it is necessary and indispensable to improve the crystallinity of the semiconductor layer (i.e., the crystalline silicon film in this case) which greatly influences the ON current above.
The crystalline silicon film above can be obtained by crystallizing an amorphous silicon film by applying heat treatment, irradiating a laser light, or by utilizing the both. In particular, the method of using laser light (said method hereinafter referred to as xe2x80x9claser crystallizationxe2x80x9d) as a crystallizing means or as a means for improving the crystallinity is effective from the viewpoint that it enables a crystalline silicon film having excellent crystallinity at a low temperature.
This method of forming a crystalline silicon film at a low temperature is advantageous in that a high performance thin film transistor can be fabricated on an inexpensive glass substrate. Accordingly, this method is surely a promising means for crystallization.
A pulse-emitting excimer laser is most frequently used in the method utilizing a laser light irradiation. The method using an excimer laser comprises emitting a laser having a wavelength in the ultraviolet region by applying a high frequency discharge to a predetermined type of gas and thereby realizing a particular excitation state.
In case of forming a crystalline silicon film by irradiating a laser light, however, there is a problem that not always good reproducibility is obtained on the crystallinity of the resulting crystalline silicon film. This is due to the influence of the parameters included in the process steps from the formation of a silicon film to the completion of laser annealing treatment.
The parameters included in the process steps are factors influencing the laser crystallization, and are uncertain factors influencing the crystallinity. They include indirect factors such as the film thickness of the amorphous silicon film and the direct ones such as the irradiation energy of the laser.
In case of an excimer laser, for instance, the presence of fluctuation in the irradiation energy per pulse of the emitted laser light is found as a problem. Furthermore, the fluctuation in the irradiation energy of the laser and the scattering in energy distribution in the superposed emissions of laser light are known to induce non-uniform crystallinity.
For example, the inventors use a laser device in which the laser is linearly beam-processed to provide laser-irradiated surfaces that are superposed on each other. Accordingly, the heterogeneity in energy distribution directly induces the fluctuation in ON current as to form transverse stripe pattern in the image display region.
As described in the foregoing, the stripe patterns provide a fatal problem in manufacturing a commercially feasible liquid crystal display device. Thus, early solution to the problem is keenly demanded. However, by employing the laser device at the present level of technology, it is almost impossible to form a crystalline silicon film having a crystallinity which induces perfectly no fluctuation in ON current, which is the cause of stripe patterns.
In other words, this problem is the rate-determining factor in the evolution of liquid crystal display device utilizing the low temperature polysilicon technique based on laser crystallization technique.
An object of the present invention is to provide a technique to overcome the aforementioned problems, which is capable of performing laser annealing with excellent uniformity and reproducibility, and to provide a device for implementing the technique. It is also an object of the present invention to provide, by applying the technique above, a technique for fabricating a liquid crystal display device capable of forming an image with high quality and free of stripe patterns.
According to one aspect of the present invention, there is provided a laser-irradiation method which comprises a process for fabricating a semiconductor device, comprising:
a first step of forming a thin film amorphous semiconductor on a substrate having an insulating surface;
a second step of modifying the thin film amorphous silicon into a crystalline thin film semiconductor by irradiating a laser light and/or by applying a heat treatment;
a third step of implanting an impurity element which imparts a single conductive type to the crystalline thin film semiconductor; and
a fourth step of activating the impurity element by irradiating a laser light and/or by applying a heat treatment;
wherein the peak value, the peak width at half height, and the threshold width of the laser energy in the second and the fourth steps above are each distributed within a range of approximately xc2x13% of the standard value.
In the light of the aforementioned problems of conventional techniques, the present inventors assumed that the non-uniformity in crystallinity becomes apparent as a result of the mixing of a plurality of uncertain factors such as the aforementioned thickness of the amorphous silicon film, etc.
Accordingly, the principal object of the present invention is to suppress and minimize the fluctuation in the parameters encountered in the process steps which directly or indirectly influence the laser crystallization process. Furthermore, another object is to eliminate the uncertain factors as much as possible, while suppressing the fluctuation in the parameters.
Referring to FIG. 1, for instance, in the fabrication of a crystalline silicon film by irradiating a pulse-emitted linear laser light, there is observed a fluctuation in the irradiation energy of the laser light (i.e., the fluctuation in irradiation energy with respect to the time of irradiation).
The data shown in FIG. 1 illustrates the fluctuation in the laser output (the laser energy or the irradiation energy) per pulse of the emitted radiation (i.e., the fluctuation in irradiation energy with respect to the passage in irradiation time). In case an appropriate beam formation is performed by using an optical system, the fluctuation corresponds to the fluctuation in the density of irradiated energy per shot on the irradiated surface.
In other words, although the irradiated energy is taken here in the ordinate, it is also possible to convert it and express it in terms of energy density. The laser output herein shows the peak value (maximum value) of the laser energy.
Referring to FIG. 1, it is to be noticed that the peak values of the laser output are distributed roughly within a range of xc2x13% of 640 mJ; i.e., the peak values fall within a range of xc2x13% of a certain standard value (optimum value). In the case of the laser device used by the inventors, the energy density for an irradiated unit area at a laser output of 640 mJ is about 250 mJ/cm2.
According to the study of the present inventors, it is known that, when laser annealing is performed with a fluctuation larger than the range above, the annealing effect becomes scattered, or the uniformity in the surface becomes impaired.
Incidentally, in case a higher uniformity must be achieved in laser annealing, the distribution range in laser output is narrowed to within xc2x12%, preferably to within xc2x11.5%, though this may have the expense of complicated control and increased cost.
Accordingly, considering the annealing of a semiconductor film with reference to FIG. 1, the fluctuation in laser output per pulse emission is constrained to within xc2x13%, preferably within xc2x12%, and more preferably, within xc2x11.5%. These constraints are particularly preferable in case of annealing a large area using a linearly emitted light of laser.
Further, to eliminate the aforementioned stripe patterns, it is also required not only to suppress the fluctuation in peak values, but also to suppress the fluctuation in various parameters related to the crystallization process, and to remove as much as possible the uncertain factors in laser crystallization.
In accordance with another aspect of the present invention, there is provided a laser-irradiation device for irradiating a laser light to a thin film semiconductor provided on a substrate having an insulating surface, comprising:
means for emitting the laser light;
a gas processor connected to the means for emitting the laser light;
a control unit for controlling the output of the laser light by detecting a part of the laser light and then feeding back the detected result to the means for emitting the laser light;
optical means for shaping the laser light into a linear beam; and
means for heating the thin film semiconductor; wherein, the peak value, the peak width at half height, and the threshold width of the laser energy are each distributed within a range of approximately xc2x13% of the standard value.
Furthermore according to still another aspect of the present invention, there is provided a laser-irradiation device for irradiating a laser light to a thin film semiconductor provided on a substrate having an insulating surface, comprising:
means for emitting laser light;
a gas processor connected to the means for emitting the laser light;
a control unit for controlling the output of the laser light by detecting a part of the laser light and then feeding back the detected result to the means for emitting the laser light;
optical system means for shaping the laser light into a linear beam;
means for heating the thin film semiconductor; and
an auxiliary heating device provided in addition to the means for heating the thin film semiconductor; wherein, the peak value, the peak width at half height, and the threshold width of the laser energy are each distributed within a range of approximately xc2x13% of the standard value.
Referring to FIG. 7, the laser device employed in the present invention is briefly described below. The laser device illustrated in FIG. 7 is necessary for providing a laser energy distributed in a range shown in FIG. 1.
Referring to FIG. 7, the pulsed light emitted from a laser generator 702 is processed into a pulse beam having a linear cross section by using an optical system 706, reflected by a mirror 707, and is irradiated to an object substrate 709 through a quartz window 708 into a laser irradiation chamber 701.
As the light emitted from the laser generator 702, usable are radiations in the ultraviolet region, such as a KrF excimer laser (having a wavelength of 248 nm), an XeCl excimer laser (having a wavelength of 308 nm), and fourth harmonics (having a wavelength of 265 nm) of a xenon-lamp excited Nd:YAG laser.
Furthermore, a gas processor 703 is connected to the laser generator 702. The gas processor 703 corresponds to an excited gas purifier for removing halides (i.e., fluorides in case of KrF excimer laser and chlorides in case of XeCl excimer laser) generated inside the laser generator 702.
A half mirror 704 is provided between the laser generator 702 and the optical system 706 above, so that a part of the laser output is taken out and detected by a control unit 705. The control unit 705 controls the discharge power of the laser generator 702 in correspondence with the fluctuation in the detected laser energy.
The object substrate 709 is placed on a stage 711 provided on a substrate support table 710, and is maintained at a predetermined temperature (300 to 650xc2x0 C.) by a heater provided inside the substrate support table 710. The stage 711 is equipped with a thermocouple 712 so that the measured result may be feed back immediately to control the heater.
Furthermore, the laser irradiation chamber 701, whose atmosphere is controllable, is equipped with a vacuum evacuation pump 713 as a means for reducing pressure and for evacuation. The vacuum evacuation pump 713 is capable of realizing high degree of vacuum, e.g., a turbo molecular pump and a criosorption pump.
A gas supply pipe 714 connected to an O2 (oxygen) gas bomb via a valve and a gas supply pipe 715 connected to a He (helium) gas bomb via a valve are provided as a gas supply means. Preferably, the gas used herein has a purity of more than 99.99999% (7N).
In the laser irradiation chamber 701 having the constitution described above, the substrate support table 710 is moved in a direction making a right angle with respect to the linear direction along the linear laser beam. This constitution allows the laser beam to be irradiated while scanning the upper surface of the object substrate 709.
A gate valve 717 is provided as an inlet and outlet for charging and discharging the object substrate 709, and is connected to an external substrate transport chamber.
Referring to FIG. 8,. the process for processing the pulsed laser beam inside the optical system 706 (shown in FIG. 7) is briefly described below.
Firstly, by using an optical system consisting of optical lenses 801 and 802, the laser light emitted from a laser generator is shaped into a laser light having a predetermined beam shape and a predetermined distribution of energy density.
The distribution of energy density in the resulting laser light is corrected by two homogenizers 803 and 804.
The homogenizer 803 has a function of correcting the energy density in the width direction within the finally obtained linearly shaped beam.
The homogenizer 804 has a function of correcting the energy density in the longitudinal direction within the finally obtained linearly shaped beam. Because the laser beam is extended in the longitudinal direction for a length of 10 cm or more, the setting of the optical parameters of the homogenizer 804 must be carried out with great care.
Optical lenses 805, 806, and 808 are provided to linearly shape the laser beam. In addition, a mirror 807 is provided.
In the constitution according to the present examples, 12 cylindrical lenses (each having a width of 5 mm) constitute the homogenizer 804. The incident laser beam is split into approximately 10 beams.
That is, the homogenizer is arranged with a little margin with respect to the laser light so that the inner ten cylindrical lenses are mainly used.
In the present example, the finally obtained length of the laser radiation energy in the longitudinal direction of the laser radiation is 12 cm.
By employing the constitution above, the unevenness in energy density of a linear laser light can be eliminated, and uniform annealing can be applied to a semiconductor material.